Tuesday, May 22, 2007

Lithium-Ion Batteries That Don't Explode

A new material prevents overheating, making lithium-ion batteries safer for use in vehicles.

By Kevin Bullis

Battery saver: Strips of this thin white film could be key to preventing battery fires of the sort that forced the recall of millions of batteries last year. The material, which separates electrodes inside a battery, changes structure to prevent overheating. Credit: Courtesy ExxonMobil Chemical

A new polymer material could prevent the type of battery explosions that led to last year's massive recalls of lithium-ion laptop batteries. (See "Safer Lithium-Ion Batteries.") By making such batteries safer, the new material could help clear the way for the widespread use of lithium-ion batteries in hybrid and electric vehicles.

Lithium-ion batteries are used in laptops because they're small and light compared with the alternatives. In cars they could replace the nickel metal hydride batteries used in hybrids now, saving room and improving fuel economy by reducing weight. But so far they haven't been used extensively in cars, in part because of safety concerns. (See "Are Lithium-Ion Electric Cars Safe?")

The batteries can explode and burst into flame when they overheat--a result of overcharging or of the electrodes inside the battery coming into contact, causing an electrical short. While a laptop fire can be dangerous, batteries for such devices only involve a few cells. A fire caused by thousands of cells in a battery pack for cars could be much worse.

Last year, millions of laptops were recalled by such major companies as Apple and Dell because metal particles were accidentally incorporated into battery cells during manufacturing. In rare cases, these particles could penetrate a plastic sheet called a separator that ordinarily prevents the positive and negative electrodes within a cell from touching. Such an event can generate heat, which can cause the separator to break down further, resulting in more shorting and more heating. At high enough temperatures, the electrode materials decompose, releasing oxygen and leading to more-rapid heating and, ultimately, an explosion and fire.

Researchers at Tonen Chemical, an affiliate of ExxonMobil Chemical based in Tokyo, Japan, have developed a new separator that plays an active role in keeping batteries from overheating. The material could make it possible to slow the reactions, allowing the battery to cool off rather than bursting into flame, says Peter Roth, program manager for advanced technology development at Sandia National Laboratories, in Albuquerque, NM. Sandia is now testing the safety features of the new separator.

Separators are electrically insulating materials that have been engineered to have pores that allow lithium ions to shuttle back and forth between a battery's electrodes while the battery is being charged and discharged. A new generation of separators are designed to soften when they reach a certain temperature, about 130 ºC. That closes the pores, shutting off the current flow. In some cases, this will stop the overheating. But if the temperature continues to rise in the cell, these materials melt completely, breaking down and causing massive electrical shorts that can accelerate heating. If the cell tops 180 ºC, the electrode materials can decompose, releasing oxygen that allows the battery's electrolyte to catch fire and the battery to explode.

Unlike these separators, which break down at a little above 150 ºC, the new Tonen material stays intact up to 190 ºC. By preventing massive electrical shorting, the new separator could prevent the accelerated heating that leads to explosions, Roth says.

The performance of the separator is due to the fact that it incorporates more than one polymer: one that softens at 130 ºC to shut down current, and another to keep the separator intact to prevent shorting. Since the material can be made by modifying existing manufacturing equipment, it could quickly be available in large amounts, according to Koichi Kono, Tonen's R&D manager. "Commercially, we are ready," he says.

Other companies have developed alternative approaches to making lithium-ion batteries safer, including using different electrode materials or nonflammable electrolytes, or adding a thin layer of ceramic material to keep the electrodes separated. While the ceramics can survive very high temperatures, questions remain about how well they can be incorporated into manufacturing processes and whether they will be too expensive, Roth says. "The goal is to have batteries that fail gracefully rather than explosively."